A coin-shaped electric double layer capacitor as shown in FIG. 2 has been conventionally known. This electric double layer capacitor includes two polarizable electrodes 1, 2 and a separator 3 intervening therebetween and impregnated with an electrolyte, which are housed in an outer packaging lid 4a and an outer packaging case 4b. A first polarizable electrode 1 is connected via a current collector 5 to the outer packaging lid 4a, while a second polarizable electrode 2 is connected via a current collector 6 to the outer packaging case 4b. The outer packaging lid 4a is caulked for sealing into the outer packaging case 4b via a gasket 7 for electrical insulation. The outer packaging lid 4a has a lower portion 41a formed with a diameter larger than that of an upper portion 40a in order to be easily caulked into the outer packaging case 4b via the gasket 7.
For electrodes used in a battery such as a lithium battery, a positive pole (cathode) and a negative pole (anode) are produced from different materials and therefore have different capacities per volume, so that the positive pole and the negative pole used therein are different in volume in order to balance the capacities. However, with the electric double layer capacitor as described above, the first polarizable electrode and the second polarizable electrode are formed from the same material and therefore have a generally equal capacity per volume. The same shape is also used because of good productivity and low production costs.
With the above-described coin-shaped electric double layer capacitor, the first polarizable electrode 1 has a diameter defined such that the first polarizable electrode can be contained in the upper portion 40a of the outer packaging lid 4a. The second polarizable electrode 2 also has the same size diameter. This has been causing a problem of useless space around the second polarizable electrode 2 arranged in the lower portion 41a, which has a diameter defined larger than that of the upper portion 40a. 
As a method for solving the above-described problem, there has been proposed a so-called bottom-laid structure in which, as shown in FIG. 3, the second polarizable electrode 2 is expanded circumferentially until it almost reaches the inner peripheral surface of the outer packaging case 4b (this is called a bottom-laid structure because the second polarizable electrode is arranged below the gasket) (see JP 11-67609 A).
When a voltage is applied to the electric double layer capacitor, cations and anions in the electrolyte are attracted to the first polarizable electrode and the second polarizable electrode, respectively. The capacitance of the electric double layer capacitor depends on how many ions the respective polarizable electrodes can attract. With the electric double layer capacitor of the above-described bottom-laid structure in which the second polarizable electrode 2 is expanded circumferentially as shown in FIG. 3, only the second polarizable electrode 2 provides an increased ion attraction amount, while the first polarizable electrode 1 provides an unchanged ion attraction amount. This has been preventing the capacitance from greatly increasing.
In view of the above-described problems, the present invention provides an electric double layer capacitor in which the useless space around the polarizable electrode is effectively utilized, and the capacitance increases more than in the conventional products.